This invention relates to a keyboard switch of a non-contact type used for a keyboard, more specifically to a keyboard switch of a capacitive-coupling type.
In a capacitive-coupling type keyboard switch (hereinafter referred to as a capacitive switch), what is called a capacitor is generally provided with fixed and movable electrodes facing each other at a distance, with a dielectric disposed between them. The movable electrode is fitted with an operating member whereby the distance between the movable and fixed electrodes can be varied.
It is generally known that the capacitance of a capacitor is in inverse proportion to the distance between the electrodes and varies in proportion to the area of the facing portions of the electrodes where the voltage applied between the electrodes is constant. In the aforesaid conventional capacitive switch, the facing area is fixed, and the distance between the electrodes is variable. Accordingly, the capacitance of the capacitive switch varies with the change of the distance between the movable and fixed electrodes, and the capacitive switch performs a switching operation according to the variation of the capacitance. Thus, if the capacitance is higher than a given value, the switch is capacitive-coupled to allow a high-frequency component of AC current to flow. If the capacitance is lower than the given value, on the other hand, the switch is not capacitively coupled, and is turned off.
In the prior art capacitive switch, the relationship between the change of the depth of depression of the operating member and the change of the capacitance, which depends on the change of the distance between the electrodes, may be represented by a hyperbolic curve, as shown in FIG. 1. According to such a relationship, as is evident from FIG. 1, the changing rate of the capacitance is low when the depth of depression is small, and increases drastically when the depth of depression exceeds a given value. Thus, the setting range for the operating value of capacitance for the performance of switching operation is quite limited, so that it is very difficult to optionally set the operating value.
For example, if the operating value is set within a range where the rate of change of the capacitance responsive to the change of the depth of depression is high, no great hysteresis can be obtained, and it is hard to prevent an erroneous second depression of the key.
Namely, the depth of depression of the operating member for shifting the capacitive switch from OFF-state to ON-state (ON-operation depth of depression) and the ON-operation capacitance corresponding thereto are generally set greater than the depth of depression of the control member for shifting the capacitive switch from ON-state to OFF-state (OFF-operation depth of depression) and the OFF-operation capacitance corresponding thereto, respectively. The differences between the ON-operation depth of depression and the ON-operation capacitance corresponding thereto, and the OFF-operation depth of depression and the OFF-operation capacitance corresponding thereto constitute the aforesaid hysteresis. The aforesaid operating value is defined by the ON-operation capacitance.
Since the difference between the ON- and OFF-operation capacitances is normally set to be constant, the difference between the ON- and OFF-operation depths of depression will be reduced if the operating value for the capacitive switch is shifted as required to a range in which the rate of change of the capacitance responsive to the change of the depth of depression is high. Thus, if hysteresis is small, the capacitive switch will cause the second depression or repeated ON-OFF operations attributable to the normal movement of an operator's finger during depression, for example. As a result, where the capacitive switch is used in an input unit or an electric typewriter, over striking or other awkward effects can occur.
Inflammation of the tendon sheath has recently been occurring in the wrists of keyboard operators using keyboards as an input means for information equipment such as computers. Such an inflammation has become an occupational disease, thus provoking social concern. Inflammation of the tendon sheath may occur due to the following reason.
Conventionally, a capacitive switch is used as a keyboard switch. The capacitive switch is electrostatically connected when an electrostatic capacitance has reached a predetermined operation value, so that a high-frequency component of an AC current is rendered conductive. The operation value is set such that the switch is turned on when the switch is depressed halfway down. The operator need not depress the operation member to the lowest position at which the member is in contact with a corresponding electrode. However, since the operator is afraid of typing errors, he or she may depress the operation member until it abuts against the electrode. This is because the switch is completely turned on when the operation member abuts against the electrode and because the abutment between the impact of the operation member and the electrode can be felt by the operator. The impact acting on the finger of the operator upon abutment between the operation member and the electrode does not entail a forceful impact on the finger when such abutments are repeated several times. However, if the operator continues key input operation for a few hours, the impact accumulates. The total impact cannot be neglected and inflammation of the tendon sheath results.